Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in castings, such as for instance turbine blades, in order to substantially increase their service life and strength, in particular the fatigue strength. Another field of application is the manufacture of products, which are required to be fully dense and to have pore-free surfaces, by means of compressing powder.
In hot isostatic pressing, an article to be subjected to treatment by pressing is positioned in a load compartment of an insulated pressure vessel. A cycle, or treatment cycle, comprises the steps of: loading, treatment and unloading of articles, and the overall duration of the cycle is herein referred to as the cycle time. The treatment may, in turn, be divided into several portions, or states, such as a pressing state, a heating state, and a cooling state.
After loading, the vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure and temperature of the pressure medium is then increased, such that the article is subjected to an increased pressure and an increased temperature during a selected period of time. The temperature increase of the pressure medium, and thereby of the articles, is provided by means of a heating element or furnace arranged in a furnace chamber of the pressure vessel. The pressures, temperatures and treatment times are of course dependent on many factors, such as the material properties of the treated article, the field of application, and required quality of the treated article. The pressures and temperatures in hot isostatic pressing may typically range from 200 to 5000 bars, and preferably 800 to 200 bars, and from 300° C. to 3000° C., and preferably from 800° C. to 2000° C., respectively.
When the pressing of the articles is finished, the articles often need to be cooled before being removed, or unloaded, from the pressure vessel. In many kinds of metallurgical treatment, the cooling rate will affect the metallurgical properties. For example, thermal stress (or temperature stress) and grain growth should be minimized in order to obtain a high quality material. Thus, it is desired to cool the material homogeneously and, if possible, to control the cooling rate. Many presses known in the art suffer from slow cooling of the articles and efforts have therefore been made to reduce the cooling time of the articles.
In U.S. Pat. No. 5,118,289, there is provided a hot isostatic press adapted to rapidly cool the articles after completed pressing and heating treatment. This is achieved by using a heat exchanger, which is located above the hot zone. Thereby, the pressure medium will be cooled by the heat exchanger before it makes contact with the pressure vessel wall. Consequently, the heat exchanger allows for an increased cooling capacity without the risk of, for example, overheating the wall of the pressure vessel. However, since the heat exchanger is located close to the top closure of the pressure vessel there is a risk that the cooling capability of the heat exchanges is impaired due to undesired heating of the heat exchanges caused by ascending thermal energy within the pressure vessel. Therefore, it may be desirable to enhance the cooling capability of the heat exchanger. It is well known within the art that an increased flow rate of the pressure medium entails an enhanced cooling due to an increased heat transfer coefficient. In U.S. Pat. No. 5,118,289, an increased flow rate is achieved by allowing the circulating gas (pressure medium) to pass the heat exchanger via a pump of fan or the like. This solution may, on the other hand, add complexity to the construction of the pressing arrangement as well as it may increase maintenance requirements and needs.
Hence, there is still a need within the art of an improved pressing arrangement for hot isostatic pressing that is capable of controlled rapid cooling of articles and of pressure medium.